One of the things I found annoying with using halftracks is that the MG only fires forward, so the entire vehicle has to stop and pivot if it sees a target. Amongst other reasons, this is annoying as it makes them easy targets for guns. Real halftracks have their guns mounted on pintels which allow all-around fire, even on the move. I decided to simulate this by giving my halftracks tiny "turrets" for their MGs These aren't really visible when playing the game, but allow the MGs to be fired in any direction on the go, without the vehicle having to stop and manuever.

It plays very well, and like I said, doesn't affect the appearance of the vehicle. To choose which turret to use, I found the smallest available in the same color as the vehicles.

The other mod I did was for the US halftrack. Instead of a LMG, I gave it a HMG to better replicate the .50 cal used on the M3A1. I lowered the ammo stored to 700 rounds, which is more accurate. I also gave the British a halftrack with the LMG. They used a lot of them. Here are the data for my halftrack mods, hope you find them useful:

However, in reality, the pintle mounts only allowed engagement in the frontal arc of fire. Neither the M2 .50 or the MG34/42 7.92mm machine guns on US or German halftracks could fire to the rear arc. The gunner would have to have been on the vehicles hood!

Thus, the "factory" armament for each had one (German) or two (US) other machineguns mounted on the sides or rear. These were not seen as often in action though.

The US halftrack came in two main versions, the M3, and the M3A1. The M3 had the .50 cal mounted on a pintel towards the front of the passenger compartment. It could be fired in any direction, which was essential due to the fact that one of the main reasons for it being carried was for AA defense.

M3 .50 mounting:

The other main version was the M3A1. It was found in use that the .50 gunner needed better protection against small arms fire when using the gun, so an armored "pulpit" was mounted in the front right hand corner of the compartment. The .50 was mounted on a circular rail inside this compartment and could be slid around it to face and fire in any direction, against ground or air targets.

M3A1 .50 mounting (click for larger view):

You're right that the 251's mg could only fire to the front 180 degrees without being dismounted. However, when a squad was on board it was supposed to mount the rear gun, or also when used in the fire support role. I still think that this is the best in game compromise, primarily since it removes the terrible and innaccurate predilection of the vehicle to stop and manuever to fire at any targets, which is completely innaccurate, while the ability of the vehicle to fire in a 360 arc was only uncommon.
Here's a pic of the 251 showing both mg mountings: